(1) Field of the Invention
The present invention relates to a biodegradable plastic composition and a method for control of biodegradation rate of biodegradable plastic. More particularly, the present invention relates to a biodegradable plastic composition wherein a carbodiimide compound is added to a biodegradable plastic to control the biodegradation rate of the plastic, and a method for control of biodegradation rate of biodegradable plastic.
(2) Description of the Prior Art
In recent years, environmental pollution caused by plastic waste has become a bigger problem. Meanwhile, the necessity for and significance of environmental protection has become more important. Under such circumstances, research and development on biodegradable plastics is under way.
Biodegradable plastics can be largely divided into three kinds; that is, a plastic whose molecular skeleton is an aliphatic polyester resin, a plastic whose molecular skeleton is a polyvinyl alcohol, and a plastic whose molecular skeleton is a polysaccharide. Of these plastics, an aliphatic polyester resin has found no practical application as a biodegradable plastic because it generally has a low melting point, is inferior in thermal stability when produced, and has no sufficiently large molecular weight and is unable to give a practical molding having satisfactory properties. However, researches were made to eliminate these problems; as a result, a high-molecular aliphatic polyester resin was developed and is finding applications, for example, as materials for agriculture, forestry and fishery (e.g. films, pots for plant cultivation, fishing line and fishing net), materials for civil engineering (e.g. sheets for water retention, vegetation nets and sand bags) and materials for packaging or containers (those to which earth, food or the like sticks easily and which are difficult to reuse).
Biodegradable plastics such as the above aliphatic polyester resins and the like are needed to satisfy the following requirements.
(1) When in use, they show properties (e.g. strength, water resistance, moldability and heat resistance) similar to those of conventional plastics.
(2) When disposed as a waste, they are quickly degraded by microbes widely living in nature.
For the above reason, several proposals were made on the control of biodegradation rate of biodegradable plastic. Of them, known are, for example, a proposal which comprises adding a hydrolysis enzyme to a biodegradable plastic to shorten the biodegradation time of the plastic [Japanese Patent Application Kokai (Laid-Open) No. 168149/1992] and a proposal which comprises reducing the content of unreacted monomer(s), impurities, chain type or ring type oligomer(s), etc. in biodegradable plastic to extend the biodegradation time of the plastic [Japanese Patent Application Kokai (Laid-Open) No. 12688/1997].
Biodegradable plastics according to the above conventional techniques, however, still have problems such as the following, and their biodegradability is not yet controlled at a satisfactory level.
(a) In producing the raw material(s) of biodegradable plastic in the form of pellets or producing a biodegradable plastic from the pellets, they are exposed to the moisture present in the atmosphere or are heated; thereby, hydrolysis proceeds in the biodegradable plastic and the initial properties of the molded product of the biodegradable plastic are deteriorated or fluctuate.
(b) The biodegradability of the product is unstable.